The present invention relates generally to fluid pressure regulators, and, more specifically, to two-stage pressure regulating valves convertible without parts replacement to separate output pressure ranges, such as those typically used in natural and LP fuel gas controls.
Applications have existed for some time for pressure regulating valves capable of regulating to either of two electrically selected pressures. In general, these applications have been somewhat specialized, and the required volumes relatively small, thus providing little incentive for exploring nontraditional designs. To the extent two-stage pressure regulating valve designs are known, they generally employ a solenoid actuator to change the force supplied by a regulator spring.
A pressure regulating valve application which is receiving increasing interest is fuel gas control for small furnaces, such as those used in single family residences, including both permanent fixed location homes and mobile homes. Certain U.S. furnace manufacturers have developed high efficiency furnace designs which rely on two stages of furnace operation. When heat is first called for, and under moderate heat requirements, the furnace is fired at a low to moderate rate. If firing at that rate does not provide the heat output required for existing conditions, firing is stepped to a higher rate.
Another factor which impacts gas valve manufacturers and users is that both natural gas and gas produced from liquefied petroleum (LP) or propane are in common use as fuel gases. These gases have different burning characteristics and are supplied to a furnace or other burner apparatus under different pressures. Specifically, gas generated from liquefied petroleum has a faster flame front. Accordingly, it is supplied to a burner at a higher pressure, e.g., 10 inches H.sub.2 O, which induces more oxygen entrainment. In contrast, natural gas is typically supplied to a furnace or burner at a pressure of 3.5 inches H.sub.2 O. Accordingly, a pressure regulating gas valve for a furnace or other appliance must regulate to a higher pressure for LP gas than for natural gas.
There are various situations in which it is not known at the outset which type of fuel gas will be used, and/or in which the type of fuel gas may be changed at some point during the operating life of a furnace or other appliance. Either situation often occurs with a mobile home. Another common situation arises in connection with new construction of permanent fixed location dwellings. Initially natural gas may not be available because underground gas lines are not in place in the area, or because hook-up to natural gas lines is delayed by frozen ground, or for other reasons. Typical solutions in the past have been to either change out the entire gas valve at the time of conversion to a different fuel gas, or to replace certain components, such as regulator springs or pressure regulator modules. Both of these solutions are undesirable for several reasons. Further, mobile home requirements now dictate that gas valve conversion be achieved without replacement of gas valve parts.
One recent gas valve design which addresses this problem employs an solenoid plunger assembly whose position along the axis of plunger movement is determined in part by a cam arrangement in a cylindrical stop mechanism co-axial with the plunger assembly. The stop mechanism includes two parts having facing ends formed at an angle different from perpendicular to the axis, so that as the first part is rotated with respect to the second part, the second part is axially displaced. The second part is mechanically coupled to the solenoid armature so as to change the unactuated and actuated positions of the armature relative to the solenoid coil, thereby changing the unactuated and actuated compressions of a regulator spring.
This design accommodates only a single low firing rate adjustment and a single high firing rate adjustment. Accordingly, the valve must be adjusted for low and high firing rates each time it is converted between natural and LP gas usage. Obviously, it would be preferable to be able to convert between usages in different gas applications without requiring firing rate adjustments.
An additional disadvantage of this design is that repositioning the solenoid armature relative to the coil for conversion between pressure ranges suitable for natural and LP gas respectively changes the air gaps in the magnetic circuit, thereby resulting in a less than optimum solenoid configuration for one or both of the ranges.
Although gas valves are available for two-stage operation, and gas valves are available which provide for conversion without parts replacement, an increasing need exists for a single gas valve which achieves both objectives, as well as permitting conversion with no or minimum valve adjustments, and providing uniform solenoid performance regardless of the pressure range for which the valve is set. The applicant has devised a design for such a valve which retains the advantageous features of prior limited purpose designs, while adding the capability of two-stage operation, no parts replacement conversion, and conversion without firing rate adjustments.